Question on PPI or Radar displays
 

OK sorry for this one of the I am reading something and wonder how questions.

Reading M. G. Scroggie 8th Edition Foundations of wireless, and have read on TV receivers and a brief section on P.P.I. radar screens. In it, it describes rotating the time base deflection coils in sequence with the aerial. That I can understand, it is mechanical rotation and allows a single sawtooth waveform applied to the scan coils to deflect the dot from centre to the edge of the screen. But how would this be done with electrostatic deflection, I am trying to think of several waveform generators and how they would be applied?

In my head I have one sawtooth waveform and two slow sine-waves (In sync with aerial rotation), one leading the other by 90 degrees, both being summed to the sawtooth with one going to the X and one going to the Y deflection plates.

So my question is am I anywhere near how it is done, are there any circuits as an example to show how it was done etc?

It could be fun to play with my scope external inputs at some point.

Cheers

Adrian

Re: Question on PPI or Radar displays
 

You make two slow sinewaves 90 degrees apart in phase. Fed to X and Y amplifiers with the gsins twiddled to match you get a basic circular scan.

Now make you sawtooth for the time=distance scan. Apply it as amplitude modulation to both the X and Y signals above and you get a radial sweep (it modulates the radius of the circular scan.)

David

Re: Question on PPI or Radar displays
 

A modified 1:1 lissajou figure then.

I suppose it beats feeding rotating scan coils via slip-rings.

Re: Question on PPI or Radar displays
 

So I had the correct idea, that is novel for me! I am at present going through old valve oscilloscopes looking at the scan and sync circuits and trying to understand how done, bed time I think.

Adrian

Re: Question on PPI or Radar displays
 

Sync.... yuk! unless you're restoring an old oscilloscope as a historical item, you're a lot better spending your effort on scopes with triggered timebases. They make more sense and operate logically over a wide range of circumstances. Much better if it's a scope you actually intend to use.

A guy called Howard Vollum invented the triggered timebase for oscilloscopes. He approached Bill Hewlett and Dave Packard, hoping to sell the idea. H & P were too busy with microwave test gear for uncle sam, that they couldn't take it up. So they gave Howard some help and encouragement in setting up his own business as they had done themselves. Howard named it 'Tektronix' So HP and Tek were on friendly terms from the beginning.

David

Re: Question on PPI or Radar displays
 

Hello David, I am not restoring any old scopes, thankfully my HP182 Frame based scope is working fine.

I can read an article or watch a video of things gone past and wonder how they did it in the days of valves. So up to now on the time base I have the old charging the capacitor via resistor and a trigger tube which could be a neon, cold cathode or a thyratron, or using a valve multivibrator circuit.

Back to the two sine wave generators to get circular motion, I was wondering about just one RC phase shifting oscillator with outputs taken across 90 and 180 degrees which could be done either by two or 4 phase shifts, two preferable. So less of an issue trying to keep two low frequency osc's in sync.

It's a brain exercise for me, that not to say I will not consider putting something in to a practical circuit at some point.
Lockdown and retirement means I have some time on my hands.

I have just bought a small CRT!

Cheers

Adrian

Re: Question on PPI or Radar displays
 

In the WW2 H2S ground-mapping radar that I am particularly interested in, the signals to drive the X and Y deflection plates of the display tube are sourced from the two outputs of a sine-cosine resolver which is mounted on the scanner assembly and whose rotor is rotated in synchronism with the aerial 'dish'. (The sine-cosine resolver is part of the Magslip family, but must not be confused with the three-phase transmitter/receiver variety).
An alternative, used in the ASV MkXI radar was a 'slab potentiometer' which I described on here a few weeks ago, the Search function should find it.

Another method which I have read about in some Marconi marine radars, did indeed use three-phase Magslip techniques - with a transmitter mounted on the scanner and a receiver stator (without the rotor) mounted on the magnetically-deflected CRT neck.

Andy

Re: Question on PPI or Radar displays
 

Hope this may be of some interest...

Re: Question on PPI or Radar displays
 

Sorry for delay had PC issues after an , I had to search what a sine-cosine resolver was and thanks for the circuit.

Adrian

Re: Question on PPI or Radar displays
 

In the 1960's/70's, the Royal Navy produced a handbook called Basic Radar where this principle was described.

I've attached screenshots of the relevant pages. I have the whole book scanned in to PDF

John

Re: Question on PPI or Radar displays
 

Thank you John for the information. 10 revs per min that is slower then I thought

I have an idea building in my head, this is a slow idea, in that I have an amplifier to build (awaiting transformers) and an AR77 to source parts for and then try to get working, but this new idea is starting to form.
I have a DG7/5 coming soon (I hope) which is electrostatic deflection, and today ordered a 6D4 Thyratron, in the hope of building a simple radar I want to try with one of the 10GHz Doppler units I have to see if I can send and receive the pulse echo. Doing it old school if I can with simple circuits, I am not sure if the Doppler unit can be pulsed or not as that will be just switching on for a brief time a 7-8 Volt DC supply to the gun diode.

Initially going for the range finder style display as in a line across the scree with the blip at a point to indicate range. I have to sit down and do some maths as I am not sure if I can do it all fast enough for the Doppler unit to be effective as they are extremely low power. The returning echo into a mixer diode may be to weak to resolve and things close up that may show could just get swamped by the firing pulse.

Just ideas, I may be daft even trying, but what have I to loose but my sanity. Blackburn could go into lockdown again so once again time could be on my hands.

Adrian

Re: Question on PPI or Radar displays
 

For example a radio signal would do a 500 meter journey in 1.66uS, so 250m out and back.

A 1uS pulse at 10GHz could be up to 100 cycles of rf, but the difference is very little for any receiver to recover, perhaps I will be better off with audio? Could always upset the local pets!

Adrian

Re: Question on PPI or Radar displays
 

I once went to a presentation at Plessey Radar on the Isle of Wight and they had actually built an AF radar with a PPI display to demonstrate the principle. It was able to detect the shape of the room and hard objects around the room.

I forget the frequency they used now but it just about inaudible to most and I think the PRF was 4096Hz. This made you aware of it even if you couldn't actually 'hear' it.

John

Re: Question on PPI or Radar displays
 

I seem to remember a Wireless World on an ultrasonic "radar" system for training purposes - no idea how many decades ago though.

Re: Question on PPI or Radar displays
 

I do not think you have any chance of that working. Try using ultrsound as suggested above. The frequencies and time delays involved are so much easier to use (and much more legal too).

Re: Question on PPI or Radar displays
 

Well I do have a sonar unit with a couple of sounders at 150KHz, but out of water these are very restrictive.

Is it illegal to build a distancing device in the amateur 10GHz band? I have an old Practical wireless Exe that used a Doppler module, granted it has a 60cm dish, but two way communications were managed at around 60 Miles and would have been better if there was someone to point to.

It's all speculation and something to aim for.


Adrian

Re: Question on PPI or Radar displays
 

I once looked at some modern cheap Doppler modules and found that they were unbelievably crude and operated just outside the amateur band and seemed to be illegal. Maybe you have better ones?

Re: Question on PPI or Radar displays
 

Yes they do operate outside the amateur bands, but can be tuned to be within the bands very easily.

I can use the down link set up I have for QO-100 to tune one quite well, in fact I used my PW Exe as a signal source when first doing my setup.

I am not sure on the wording within the band plan as there seem to be conflicting comments around 10GHz

10,352.5-10,368 MHz Wideband modes (Note-2)
Note 2 Wideband FM is preferred between 10,350-10,400 MHz to encourage compatibility with narrowband systems

I would call a Doppler unit wide band as it is not frequency locked and can drift a few hundred KHz over time.

But getting back on topic. A Doppler unit would be very hard pressed I admit to using pulsed TX as the mixer diode will use the outgoing rf to bias the diode and I believe mix with the reflected signal to give the DC to audio note when detecting movement, so to receive you have to have the transmit side active. What the PC Exe and other simple WBFM transceivers did was use an If of either 10.7 Mhz or 100MHz and obtain demodulation from that so when having a full duplex conversation between stations their TX frequencies were typically locked to each other with AFC one being High and the other low.


This is just me thinking now on the matter but the only way that comes to mind on using a doppler head would be :-

a) The only way I could see the Doppler units being used would be use the TX section pulsed and use a standard satellite LNB for receive. But hardly old school.

b) Use the full Doppler unit, keep it in permanent transmit so it biases the mixer and provides an 'IF' for receive and have a short modulation pulse at 10.7 MHz then the echo would provide a detectable pulse at IF frequencies? Of course lower IF's could be used.


Adrian

Re: Question on PPI or Radar displays
 

In the "Battle of Britain Bunker" http://battleofbritainbunker.co.uk/ the museum section has a Chain Home radar on display with a demonstration of radar using sound in front of it, works well.

Re: Question on PPI or Radar displays
 

Whilst waiting for some transformers to arrive I have been keeping my self occupied trying to understand circuits, build some little bits and figure why they do not work!

To this end I have started on a simple fixed sawtooth generator for the X axis of a simple display.
Line up is a 6D4 (had to buy that) CV4058 (EC90) as a cathode follower and ECC85 that I am hoping to use as a see-saw phase inverter for my CRT deflection plates. I tried to drive it directly from the 6D4 but not enough umph and the resulting waveform was very rounded hence the EC90.

The timing shown is a 45 uS ramp from 26 to 64 Volts, the flyback at 2 uS. The next bit would be to figure out how I would do blanking on the return pule and also use it to pulse something on.

It is good, although frustrating finding things will not work as expected and figure out why. Seems to be the only way for me to understand things.

Adrian